The commercial making of ductile iron was advanced considerably by the discovery that the presence of controlled amounts of magneisum or cerium would facilitate nodulizing the graphite structure and by the discovery that certain inoculants can increase or refine the graphite distribution. The prior art is well aware that the power of the nodulizing agent, when combined with molten iron to effect nodular solidification, will fade the longer the combination is held in the molten state. Thus, it has become desirable to treat the molten iron later in the casting sequence. In certain cases, the treating agent is deposited as a supply of granular material in a special chamber of the gating system of the mold into which the molten metal is to be poured. Thus, the molten metal will encounter the treating agent just before it enters the solidification cavity of the mold. In an extreme application of late treatment, the mold cavity walls may be coated with the nodulizing agent.
One of the most critical problems encountered in late metal treatment is the inability to obtain uniform dissolution of the treating agent within the molten iron. This is due in part to the dynamics of introducing a highly reactive agent to a moving body or stream of molten metal. The problem is also related to the very short time duration for introducing the treating agent in this technique and to the increasing desire to use higher concentrations of magnesium in the treating alloy to facilitate a faster nodulizing effect. Unfortunately, very high non-homogeneous concentrations of magnesium tend to promote (at higher levels) disruptive influence as a result of the reaction during introduction and thus decrease the ability to obtain uniform dissolution.
One particular prior art approach to late metal treatment has been to define an intermediate chamber in the gating system and in which is deposited a predetermined and measured quantity of granular treating agent. The flow of molten metal is diverted to enter this chamber for reaction and thence to the solidification cavity. Most often, such granular material whether loose or briquetted, is affected by the flow of the molten metal therearound causing numerous undesirable effects: (a) some drag-through of the granular material caused by the swift flow of molten metal resulting in the treating agent being trapped within the molding cavity in the unreacted condition, (b) penetration of the molten metal through certain interstices of the granular supply of treating agent, the supply thereby not being gradually and uniformly reacted with the flow of the molten metal and thus causing a non-homogeneous casting, (c) a likelihood that in high volume casting procedures, the proper amount of treating agent is not consistentlly maintained in each of the casting runs, (d) impurities and defects appear in the castings resulting from segregation present in the treating agent when high magnesium contents are employed, (e) contamination of the treating agent during storage by oxidation, (f) difficulty in recycling a treating agent in a pure state if the casting run is cancelled, (g) inability to maintain uniform shape and grain size during handling, (h) less than optimum casting yield.
What is needed is a treating agent which is shaped so that it will consistently provide uniform dissolution into molten metal flowing therepast, can be economically manufactured without the presence of segregation even though containing a high content of nodulizing agent and is unified so that it does not require measurement to be introduced at the time of casting. This need has, in part, been met by the invention of applicants disclosed in co-pending U.S. application Ser. Nos. 584,563 and 584,564, each commonly assigned to the assignee of this invention, the disclosures of which are incorporated herein be reference. These references disclose the unique advantage to be obtained by using a cast-to-shape solid block of treating agent in late metal treatment. What has not been answered by such referenced disclosures is how to make a more reliable homogeneous solid alloy block by techniques which require less capital and operating costs and yet allow some flexibility in the use of molding materials. A more reliable homogeneous solid alloy block would be characterized by the substantial absences of segregation resulting from use of proper chill rates the absence of oxidation interiorly thereof in the mass utilized, the absence of organic or refractory impurities resulting from processing carryover. Lower capital costs would be characterized by higher density casting capability permitted by close nesting of castings in a given mold, avoidance of special mold making and curing equipment, and avoidance of permanent mold destruction and particularly promoting easy recycling of molding materials. Lower operating costs would be characterized by elimination or reduction of casting clean-up, allowance of a faster pour rate from a molten metal reservoir, and use of more economical chill materials to solidify the castings.